The invention relates to an arrangement for automatic range finding, particularly for sharply focusing a motor-driven objective lens.
The present invention also relates to a photoelectric transducing unit and a system for detecting the sharpness of the image of object by means of the unit, more particularly to the disposition of a photoelectric transducer in the photoelectric transducing unit consisting of at least two photoelectric transducers whoes output characteristics are different from each other and whoes photosensitive layers preferably exist in the same image forming plane as well as to a system for detecting the sharpness of the image of object by means of the photoelectric transducing unit, especially to a system adequate for the focus detection and/or the automatic focus adjustment in the optical instrument presenting an image forming optics.
Modern cameras furnish automatic exposure measurement devices and facilities for setting the diaphragm and/or the exposure time. However, no camera on the market successfully furnishes sharp automatic focusing.
Automatic range finding methods are known, but implementing them is so costly that they are completely unsuitable for cameras. One known automatic objective focusing method involves determining the range by means of radar or optical goniometry and setting the objective to the determined range by a motor. Apart from the fact that the costs of the equipment are rather high, automatic and continuous check of the definition achieved is generally not possible.
This last mentioned drawback is avoided in a recently developed special optical system. Here the signal controlling the motor drive is generated by means of a photocell which receives light deflected by a mirror from the optical path of the objective. In this arrangement an annular reflecting mirror deflects the marginal rays of a special objective onto an axially oscillating photocell. During the oscillation of the cell through the range of maximum image sharpness, the voltage generated by the photocell in dependence on the light density passes through a flat maximum. To obtain automatic readjustment of the definition the position of the maximum is used by a phase detector which receives a reference signal in the form of the a-c voltage required for the mechanical oscillating drive.
But this method can only be used with an objective where the front and rear elements are arranged sufficiently far from each other to provide room between them for the above mentioned annular reflecting mirror. The latter only deflects marginal rays with extremely low depth of focus, to the photocell. In the photocell, these low depth of focus marginal rays cause a light density which traverses a flat maximum in dependence on the image sharpness, hence with the photocell moving in the direction of the rays.
The mechanism for generating the oscillating movement of the photocell is costly and susceptible to trouble and makes the use of complicated phase detectors etc., an absolute necessity. However, this solution is not applicable to usually existing objectives where close front and rear elements constitute parts of an assembled objective.
In the conventional disposition of the two photoelectric transducers reacting to the contrast of the image formed by an image forming optics it is requested that a semipermeable light beam splitter should be disposed in the optical path between the objective lens and the photoconductive layers of the photoelectric transducers in such a manner that the light beam coming through the objective lens and reflected by means of the light beam splitter should reach the first photoelectric transducers while the light beam coming through the objective lens and passing through the light beam splitter should reach the second photoelectric transducer. Hereby the photosensitive layers of the both photoelectric transducers are generally disposed in the image planes of the two images formed by the light beams projected by means of the objective lens and splitted by means of the light beam splitter.
For this kind of disposition such disposition has already been known that the one photoelectric transducer is composed of for example, photosensitive layer zones connected in series (series type photoelectric transducer) while the other photoelectric transducer is composed of for example, photosensitive layer zones connected in parallel (parallel type photoelectric transducer). In this kind of disposition, by connecting these photoelectric transducers in for example, a proper bridge circuit it is possible to obtain a signal which assumes a limit value when the contrast of the fine parts of an image is maximum, namely the image planes correspond with the photosensitive layers of the photoelectrical transducers.
However, this kind of disposition is not only expensive but also needs a comparatively large space as well as a fine careful adjustment. In view of the fact that two sets of a pair of this kind of photoelectric transducers are needed in order to automatically adjust the focussing of an objective lens not presenting a movable part together with the direction of the objective lens, the disposition of at least three semipermeable light beam splitters presenting four photoelectric transducers to be adjusted very carefully becomes remarkably complicated.
The space hereby needed would be large in size out of discussion as compared with the ordinary size of the camera of today.
In case a pair of this kind of the photoelectric transducers presenting a semipermeable light beam splitter is adopted for the automatic focus adjustment of the image forming optics, the mechanical vibration becomes necessary for the photoelectrical transducers or the auxiliary reflecting body or the prism disposed in the optical path of the optics with the result that in this case also the problem about the necessary space and the adjustment takes place.
On the other hand when a passive automatic focus adjustment equipment or a focus detecting equipment for an optical instrument such as camera by making use of such photoelectric transducers as mentioned above it is requested that the equipment should operate with sureness over a wide range of the brightness covering several ten times. What is at first to be solved basically is the problem as to how to stabilize the initial value of the measured value disturbed by the variation of the offset voltage of the amplifier for amplifying the signals to be measured or the noise output due to the constructional difference of the photoelectric transducers themselves.
Namely, the signal voltage generated by the photoelectric transducers reacting to the contrast of the image is necessarily small as the level of the intensity to be measured (due to the non-linear effect) especially in case the contrast of the image is low, a comparatively high amplification becomes necessary. However, a desirable effect of such a high amplification as mentioned above can be expected much of only when a signal with sufficiently high S/N ratio (ratio of signal to noise) is put in the amplifier, namely the above mentioned noise voltage is sufficiently small (at least -20db) as compared with the signal voltage to be measured which is thought the effective signal, so that the above mentioned problem as to how to stabilize the initial value of the intensity to be measured should be solved basically.
An object of this invention is to avoid these shortcomings.
Still another object is to improve automatic focusing systems.
Another object of the present invention is to provide an arrangement for automatic focus detection where commerical objectives can be used and which can be realized with a minimum of circuit and material expenditure.